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Lecture 10

Biology 2483A Lecture Notes - Lecture 10: Gastrointestinal Tract, Parasitoid, Whitefly

Course Code
BIOL 2483A
Hugh Henry

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Symbionts: Organisms that live in or on other organisms
More than half of Earth's species are symbionts
Our own bodies can be a home to many other species
A parasite consumes the tissues or body fluids of the organism on which it lives (the host).
Pathogens are parasites that cause diseases (an abnormal condition affecting the body of an
Parasites typially har, ut do’t iediately kill, the organisms they eat (unlike predators)
Degree of harm varies widely
Compare: the fungus that causes athlete's foot and Tersinia perstis, the bacterium that causes the
Parasite Natural History
Parasites typically feed on only one or a few host individuals
Include herbivores such as aphids or nematodes that feed on one or a few host plants
Parasitoids: insects whose larvae feed on a single host and almost always kill it
Macroparasites: Large species such as arthropods and worms.
Microparasites: Microscopic, such as bacteria
Most species are attacked by more than one kind of parasite; even parasites have parasites.
Many parasites are closely adapted to particular host species.
This specialization helps explain why there are so many species of parasites.
Ectoparasites live on the outer body surface of the host.
Endoparasites live inside their hosts, within cells or tissues, or in the alimentary canal.
Many fungi are ectoparasites. More than 5,000 species of fungi attack crop plants.
Mildews, rusts, and smuts grow on the surface and extend their hyphae (fungal filaments) into the
plant to extract nutrients from its tissues.
Plants are also attacked by animals
Ectoparasites: Aphids, whiteflies, scale insects, nematodes, beetles, and juvenile cicadas.
They can be thought of as both herbivores and parasites.
Animals also have many ectoparasites
Athlete’s foot fugus, fleas, ites, lie, ad tiks.
Some of these parasites also transmit disease organisms.
Many disease organisms are endoparasites
The alimentary canal is excellent habitat for many parasites. Most do not eat host tissue, but rob
the host of nutrients.
Tapeors attah to the host’s itestial all ad asor digested food.
Many endoparasites live in the host's tissues/cells
o Yersinia pestis, the bacterium that causes the plague.
o Mycobacterium tuberculosis, the bacterium that causes tuberculosis
Plants also have endoparasites
Bacterial pathogens cause soft rot; fungi can rot various plant parts from the inside out.
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Ectoparasites can disperse more easily than endoparasites.
Endoparasites have evolved various mechanisms for dispersal, including complex life cycles and
enslaver parasites.
Some parasites of the alimentary canal are dispersed in feces.
Ectoparasites are more exposed to predators, parasites, and parasitoids.
o Example: Aphids are eaten by many birds and insects, and attacked by parasites and
Endoparasites are protected from the external environment, and have easy access to food.
But they can also e attaked y the host’s iue syste.
Defenses and Counterdefenses
Parasites exert strong selection pressure on their host organisms, and vice-versa
Hosts have adaptations for defending themselves against parasites, and parasites have
adaptations for overcoming host defenses
Host organisms have many kinds of defense mechanisms.
Protective outer coverings include skin and exoskeletons.
May parasites that do gai etry are killed y the host’s iue syste.
Verterate iue systes hae eory ells that a reogize iroparasites fro
previous exposures.
Other immune system cells engulf and destroy parasites or mark them with chemicals that target
them for later destruction.
Hosts can regulate biochemistry to deter parasites.
o Example: Vertebrate hosts have a protein called transferrin that removes iron from blood
seru ad stores it so aterial ad fugal edoparasites a’t get it.
But some parasites can steal iron from the transferring
Plants also have defense systems:
Resistance genes.
Nonspecific immune responses such as antimicrobial and antifungal compounds.
Cheials that stiulate depositio of ligi, hih akes a arrier to stop a iader’s spread.
Cheial sigals that ar eary ells of iiet attak
Plants have many chemical weapons called secondary compounds.
Some animals eat specific plants to treat or prevent parasite infections.
Example: Woolly bear caterpillars switch from their usual food plants to poison hemlock when
parasitic flies lay eggs on their bodies.
Chimpanzees infected with nematodes seek out and eat a bitter plant that contains chemicals that
kill or paralyze the nematodes (Huffman 1997).
Some hosts can encapsulate endoparasites, or their eggs, to make them harmless.
Some insects have lamellocytesblood cells that can form multicellular capsules around large
objects such as nematodes.
The parasites are under strong selection pressure to develop counterdefenses:
Parasite counterdefenses:
o Ectoparasites must penetrate external defenses and toxic compounds produced by plants.
o Endoparasites must cope with defenses found inside the host.
Parasitoid wasps that attack fruit flies avoid encapsulation by injecting virus-like particles that
infect the lamellocytes and cause them to self-destruct.
Others lay eggs oered ith filaets that eoe eedded i the host’s fat ells here they
are not detected by circulating lamellocytes.
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